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I'm considering solutions for a particular high-current switching application. My preference is to use dual-coil latching relays. The only such relays which meet the power requirements are SPST configuration.

Among my many design constraints is a requirement that the switching cannot occur under live conditions. However, as I'm using high-current relays I must ensure a minimum contact (wetting) voltage/current (100mA @ 5V in my application).

The switched loads and supplies are both variable. The supplies can range up to 120V or 33A, although there are no guarantees except that the supplies will not be live during switching. There may or may not be a load present during switching. The coil supply is 24Vdc.

I'm curious what ideas everyone has to ensure my minimum wetting current requirements. My best idea at present is to use an ancillary non-latching relay operated by the same control pulse to switch the 24Vdc (used to drive the relays) across the power relays' contacts during switching (current limited, of course). It goes without saying that there are drawbacks to this solution.

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  • \$\begingroup\$ What kind of environment is the relay installed in? Wetting current requirements depend on how contaminated the contacts can get. If there is no source of contamination, there is no need to ensure a wetting current. If it's installed exposed to salty sea air the situation is totally different than if it's installed inside a climate controlled residence, for example. If this is a big concern in your system, consider using a hermetically sealed relay. \$\endgroup\$ – vofa Feb 14 '18 at 19:47
  • \$\begingroup\$ I have not found any hermetically sealed relays which meet the requirements. This is for testing/qualification so it is desirable to prevent any increase in contact resistance, though I suspect you are right insofar as in our case this is a problem not likely to manifest itself for years after the relays are installed. In either event "assume it won't be a problem" isn't really an ideal response, and I need to have the resulting solution in service long before we'd have time to test this empirically. \$\endgroup\$ – Marshall Eubanks Feb 14 '18 at 20:31
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I don't see a problem with doing absolutely nothing. If your contact switches and a load isn't connected and then, some time later, it gets connected, if the relay contact is a bit "dry" it will "wet" when at least 5 volts appear across it then behave normally.

If you were trying to switch loads that run from less than 5 volts it might be a problem of course.

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  • \$\begingroup\$ No, the wetting behavior depends on micro-arcing, which depends on there being sufficient current driven by sufficient potential at the moment of switching. Unless someone can provide references which convince me that my understanding is wrong... \$\endgroup\$ – Marshall Eubanks Feb 14 '18 at 17:53
  • \$\begingroup\$ Link to the relay? \$\endgroup\$ – Andy aka Feb 14 '18 at 18:00
  • \$\begingroup\$ www3.panasonic.biz/ac/e_download/control/relay/power/catalog/… \$\endgroup\$ – Marshall Eubanks Feb 14 '18 at 18:06
  • \$\begingroup\$ The parameter you refer to is the minimum load and doesn't necessarily imply that it needs a wetting current for loads greater than this whether the contact is made or not. \$\endgroup\$ – Andy aka Feb 14 '18 at 18:13
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    \$\begingroup\$ "Switching load" means load during switching. Otherwise it would just be "load", which of course has no minimum when the relay is idle; it can be in a completely powered-down system. The reason for specifying minimum switching current is always to ensure wetting behavior. \$\endgroup\$ – Marshall Eubanks Feb 14 '18 at 18:26

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